I started a Thay campaign that was similar. The "adventure path" was set up so that the PCs would be working for the Red Wizards and through reputation via getting jobs done and a wizard PC with family connections would be able to enter into the existing power structure and do all the things that red wizards and their peers do. The true trick, I think, is getting players who can think like achievers rather than lackeys. Most PCs will sit around until some NPC gives them something to do, and getting PCs into the habit of working for themselves is the hard bit.
It also takes a different type of game. Saving the world is easy, you just kill the guy trying to take it over. Taking it over becomes much more of a game of resource managment and becoming the targets. Even if moving up in the ranks of an existing power structure without opposition by say being the heirs in a LE (or LN or LG) kingdom, as the PCs gain power, they're going to want to use that power. Quickly, the game will be more than just the PC party because they can start calling up 50 men units to do their dirty work for them in some cases. Why go into a dungeon by themselves when they can easily order 1000 men to march with them, set up camp, and dig the entire dungeon out of the ground. Needless to say, it will probably take war and large scale troop movements to conquor the world anyway. As they gain contorl over the resources of the land the take over, they're going to want to spend it on magic items. Money guildlines will go right out the window and I don't think they apply to heads of state who can afford whatever they want.
I think they typical DM (or at least many of the ones I've had) will balk at giving such power to the PCs. They'll find themselves taking over the country, yet not having any more money or power than they would normally. To properly run such a game where the PCs are taking over the world, I think the DM will have to get used to the idea that the PCs are character of privledge and will have money, servants, and political power well beyond the typical adventuring group. Of course, the party is now also a target and will have to face troubles they normally wouldn't. Other kings will send adventuring groups to stop them. Politics and relations with organizations such as the churches, nobles, or societies (secret or otherwise) will have to be dealt with because they'll need their support.
To the point of having evil parties, that pretty much fits about 80% of the campaigns I've ever been in (and the others were only 'good only' due to DM fiat). No big deal. Same moduels usually work. Instead of doing it for alturistic reasons, the PCs are doing it to kill things and take their stuff. Even the adventures that expect PCs to do things because they're good, still provide an opportunity for the PCs to be come rich enough to sate neutral or evil PCs.